aww
Explanation
The comic is titled "Deceptive Conversation Tip" and presents the following scenario: the archaic English word "aweless" means "failing to inspire awe." In the comic, one person tells another, "Wow! That graduation speech was absolutely aweless!" and the recipient responds with "Thank you so much!" clearly interpreting it as a compliment.
The humor relies on the gap between modern English intuition and archaic word meanings. In contemporary usage, most people would hear "aweless" and assume it is a positive term related to "awe" or "awesome." But the word actually means the opposite: lacking in awe-inspiring quality, essentially calling the speech unimpressive and uninspiring. The recipient takes it as praise because it sounds like a compliment, while the speaker is actually delivering an insult disguised as admiration.
This fits a recurring SMBC format of "tips" that exploit linguistic loopholes to be technically honest while being socially deceptive. The joke plays on the English language's many archaic or counterintuitive word formations where the suffix "-less" negates the root word (as in "careless," "homeless"), but the listener fails to parse it correctly because "aweless" has fallen out of common use.
The hover text ("Of course they'll never realize, but in your heart you'll have the linguistic upper hand") completes the joke by acknowledging that this linguistic trick provides only a private, petty satisfaction, since the target will never understand they have been insulted.